The present invention relates to a display device having a display element array obtained by aligning display elements such as light-emitting diodes in a matrix and, more particularly, to a display device in which a module driver for driving the display element array can be easily mounted since its circuit arrangement is simplified, thereby achieving low power consumption and high integration of the circuit.
Conventionally, there are two drive methods for a display device having a display element array obtained by arranging display elements such as light-emitting diodes (LED) so as to display an alphanumeric pattern, a kanji pattern, a special symbol pattern, a graphic pattern or the like:
(1) A dynamic scanning method in which each display element is sequentially scanned in the same manner as in TV scanning; and
(2) A static scanning method in which a memory element is arranged for each display element, and each display element arranged at an intersection between a row line and a column line is independently driven by an electrical signal from the memory element.
In the dynamic scanning method, particularly when LEDs are used as display elements and the number thereof is increased, the ON time of each element is shortened. This is because the response speed of the display elements is very fast. As a result, the dynamic scanning method has a disadvantage in that the display luminance is degraded under the condition of the same current. The static scanning method also has a disadvantage in that the matrix wiring for arranging the memory elements in a matrix form is complicated.
In order to eliminate drawbacks of both the dynamic and static scanning methods of the display matrix array and to utilize the advantages thereof, the line sequential scanning method as a composite method of the static and dynamic methods can be effectively used. According to the line sequential scanning method, a drive signal applied to a row line of the display element array is processed by time division and is used to sequentially scan the row lines. At the same time, pixel data supplied to the column lines is selectively switched in synchronism with the time division.
According to the line sequential scanning method, however, when a screen size is increased, it is difficult to scan display devices at a frequency which does not cause flickering because of the number of scanning and the time for scanning. Such a drawback occurs in display devices such as a multicolor LED display device (64.times.64 pixel matrix) described in "Denshi Zairyo" (Electronic Material), PP 68-72, February 1980, TV scanning matrix display devices (96.times.64 pixel matrix, and 160.times.112 pixel matrix) described in "IEEE Transaction on Electron Devices", PP 1182-1186, Vol. Ed. 26, No. 68, August 1979. In the multicolor display device (64.times.64 pixels), for example, the number of pixel data is 128, and the number of scanning lines is 64. Assume that pixel data is written in each memory in units of 8 bits. Sixteen writing operations must then be performed. Therefore, 1024 (16.times.64) writing operations must be performed for one frame. A repeat frequency must be more than 100 Hz to avoid flickering. The scanning frequency must be more than 102.4 kHz (1024.times.100). In a device such as a microprocessor to which a display device of this type is coupled, a data transfer speed is about 100 kHz, which corresponds to the maximum number of pixels used in the line sequential scanning method. An instantaneous current flowing through the display element array is determined by the number of pixel data supplied to the column lines. A surge current then flows through the row lines. As a result, a flat display device of this type cannot be made compact and cannot be directly coupled to an integrated circuit which does not allow flow of a surge current therethrough. Furthermore, the luminance of the display image is degraded.
In order to provide a display device which has a large number of pixels, that is, a large screen, a flat panel display is proposed in "Conference Record of 1978 Biennial Display Research Conference" October 24 to 26, SID PP 20 to 21, 1978. More particularly, unit display devices each having a drive circuit on the lower surface of the substrate are coupled to each other. The drive circuit of the unit display device has memory elements which respectively correspond to pixels of the display element array, so that each display element array can be individually driven. As a result, the flat panel display is very suitable for the response characteristics of LEDs and can be readily arranged together with an IC.
This display device is schematically shown in FIG. 1. A unit display device 3 comprises an LED array 1 and a module driver 2, which latter is integral with the LED array 1 and provides a display function by itself. The LED array is a display section in which a plurality of LEDs of a matrix array constitute predetermined pixels on a substrate in a monolithic or hybrid structure. The module driver 2 is a drive circuit for driving the LED array 1 in accordance with the line sequential scanning method. As shown in FIG. 2, the unit display devices 3 are arranged in a matrix form to constitute a unit panel 4 which has a desired size. The unit panel 4 receives various signals and a power source voltage from a unit driver 5. The unit panel 4 and the unit driver 5 thus constitute a display unit 6 which has an overall display function.
The present inventors have proposed a detailed arrangement of the module driver of the unit display in Japanese Patent Application No. 55-78940. In principle, serial pixel data supplied to the module driver is converted to parallel data which is then stored in a static RAM in response to an address signal from the unit driver. In synchronism with data read out from the static RAM, the row lines of the LED array are scanned. Now assume that the number of elements in the row direction is m, and that the number of elements in the column direction is n. The static RAM has m.times.n bits (e.g., 16.times.16 bits). The construction of the display device is complicated when both row and column address registers are considered for accessing the RAM, thus preventing a compact module driver.
Furthermore, in the arrangement described above, the unit driver must supply various signals to each module driver. These various signals include pixel data, a clock signal, a reset signal, a parallel multibit address signal, and a select signal for selecting the read and write operations of the RAM, that is, the data storage and retrieval (display) operations. For this reason, if up to several tens of unit display devices are connected to each other, the above-mentioned arrangement is effective. However, in the case of a large screen of 30 (column direction).times.30 (row direction) unit displays, the unit driver must be arranged on a large scale since the number of bits of the address signal is increased. As a result, complex wiring must be performed between the unit driver and the unit display devices, thus resulting in inconvenience in practice.